1. Field of the Invention
The present invention relates to the field of image signal processors for processing an image as electric signals.
2. Description of the Prior Art
A copying machine wherein an original image is read photoelectrically by a solid-state image pickup device, typically a CCD (Charge Coupled Device), and image recording is performed based on thus read-out signals, that is, a so-called digital copying machine, has been proposed. Some copying machines provide image file and image transmission functions in addition to the copying functions such as above.
Many machines of this kind are those for regenerating the gradation characteristic of a halftone image, typically of a photograph, in the original artificially by the dither method, and it is generally known to perform the so-called gamma correction by properly selecting from among threshold values forming the dither matrix for the 1:1 correspondence between the density of the original and the density of the reproduced image.
FIG. 1 illustrates the image reproduction characteristics according to the above gamma correction. Area 1 shows a density detection characteristic (reader characteristic) in relation to the original density D.sub.0 and the output V of a solid-state image pickup device, and there an example is shown that an analog signal of 0 V is generated when D.sub.0 =0.1 and one of about 1.0 V is generated when D.sub.0 =2.0 (that is, black) is shown. Area 3 shows a printer characteristic which is the relationship between the number of dots K per unit area (hereinafter referred to as the cumulative frequency) and the density Dp of the reproduced image, and it is shown that by printing 16 dots in a unit area the density Dp becomes 2.0 (that is, black) and that image reproduction becomes possible at the density of Dp=0.1 by printing no dots. Here, an example showing that as many as 16 levels of density can be obtained by the use of a 4.times.4 dither matrix is given. Area 2 shows a dither characteristic, and shows a curve to determine the number of dots (up to a range of zero to 16 dots) which should be printed for achieving the image reproduction of the density equal to the original density for a density signal detected within the output range from 0 V to 1.0 V of the solid-state image pickup device.
Accordingly, now, when an original density Dd (shown in FIG. 1) has been detected, it is known how the density of reproduced image equal thereto can be obtained in the area 4 by following an arrow.
On the other hand, if the density relationship between a desired reproduced image and the original is given in the area 4, any desired dither characteristic (gamma correction curve) in the area 2 can be set considering the reader and printer characteristic curves of the areas 1 and 3.
Generally, for realizing the density of 16 gradations by the dither method, an analog video signal indicating the original density is converted to a digital value, and binary data indicating whether or not dots are to be printed is formed by comparing the resultant digital value with the threshold values (dither pattern threshold value group) stored in a advance in a memory. That is, the analog signal is converted to a binary signal. Here, for the gamma correction using a curve required in the area 2, it is necessary that a video signal be digitized by the resolution of at least the reproduced image density levels (16 levels in this example), and the larger the number of levels, the more abruptly changing a curve in the area 2 can be realized in the digital fashion.
As can be seen for the characteristic in the area 2, however, even if digitization is made to a great many levels, the output change factor in the intermediate area of 0 to 1 V is small so that there is no favorable effect of digitizing to many levels, and providing a multi-level A/D converter is of little benefit.